Composite chromium electroplate and method of making same



Feb. 3, 1959 M. WEINBERG ErAL 2,871,550

COMPOSITE OHROMIUMELEOTROPLATE AND METHOD OF MAKING SAME Filed Jan. 1o, 195s Carraqzbn d 72; O 6'7 7.747@ (0.00! -a/ 71H7) fw 7.7422 Cr 7722;@ ,B 61a? -az M17) EEE-E Cr faeE (Jess 7739471 aozmi) u 5*-2 mik Y///////// rE-4r- Carraszlan pizz ,Z456 Meid/S, ze, frz, l@ ,2f/55,12%.

United States Patent COMPOSITE `CHROMIUM ELECTROPLATE I METHOD F MAKING SAME Max Weinberg, Detroit, and Henry Brown, Huntington Woods, Mich., assignors -to The Udylite Research Corporation, Detroit, Mich., a corporation of Michigan Application January 10, 1958, Serial-No. 708,267

18 Claims. (Cl. 2li-183.5)

mil) deposit of bright chromium electroplate, or it consists of copper-nickel-chromium electroplate of about 0.5 to l mil of copper, and 1 to 1.5 mils of nickel and a final thin bright chromium plate of about 0.01 to 0.02 mil thickness. It is well-known that the corrosion protection of such composite coatings is primarily attributable to the nickel layer, and the degree of protection is dependent mainly on the thickness of the nickel layer. Further-Y more, it is now well-known that the thin iinal layer of the bright non-tarnishing 4chromium plate actually decreases the corrosion protection Value of bright nickel plate underneath it, and the outdoor exposure results with such chrome plate in an industrial or marine atmosphere leaves much to be desired. This invention is based on the discovery of a composite chromium-nickel-chromium electroplated coating having certain critical thickness relationships between the successive layers thereof which provide an unexpectedly superior corrosion-resistance while employing substantially less nickel than normally used for outdoorcorrosion protection. V l

In accordance with this invention, it has now been found that by using a thickness of chromium plate of 0.03 to about 0.2 mil thick underneath 0.2 to 2 mils thick nickel plate followed by a final bright chromium plate of 0.005 to 0.1 mil thickness, it ispossible to o'btain extraordinary corrosion protection in outdoor ex- 'f posure for industrial metals susceptible to katmospheric corrosion such as steel, iron, aluminum and itsalloys, magnesium and its alloys, copper, zinc aiidlits alloys, brasses, etc.

For example, it has now been found that by using about 0.1 to 0.15 mil of chromium plate underneath about y0.5 to 1 mil of nickel plate followed by a nal bright chromium plate of 0.03 to 0.06 mil thickness, it is possible to obtain outstanding corrosion protection in outdoor exposure compared to 2 mils nickel plate with the'usual thin chromium plate. With thinner composite coatings of this invention, for example, the composite coating consisting of 0.03 mil chromium underneath 0.2 mil of nickel followed by thin (0.005 to 0.02 mil) final chromium plate, the corrosion protection obtained is superior to copper-nickel or nickel plate of 0.5 to 0.7 mil thickness with the same final chromium plate exposed in an outdoor industrial atmosphere. Using only 0.2 mil nickel with 0.1 to 0.2 mil of chromium plate underneath and 0.03 mil of final chromium plate, the outdoor exposure results obtained were outstanding compared to l mil thick nickel plate with the usual 0.01 mil of iinal chromium plate.l On the basis of many tests it was found that the first chromium plate should bey at least 0.03 mil thick and that the minimum thickness for the nickel plate overlyingy for 20 hours.

v of the panels.

2,871,550 Patented Feb. 3, lass bright chromium plate could Vary' from 0.005 to 0.1'mil thickness and still have consistently excellent outdoor corrosion results. There is no need for'thicknesses of nickel greater than 2 mils sandwiched between the chromium plates, to obtain outstanding outdoor corroson protection, in even the most severe conditions of outdoor exposure in an industrial atmosphere. In fact, as already mentioned, 1 mil thickness of nickel sandwiched between 0.1 mil of underlying chromium and 0.03 mil of iinal chromium is superior to 2 mils of nickel with the usual nal thin chromium plate. The use of more than l0.2mil of chromium underneath the nickel is not necessary, and does not add any substantial improvement in outdoor corrosion protection in the composite coatings of this invention. Thus, this invention encompasses the use of a composite electroplate consisting of, chromium plate of 0.03 to about 0.2 mil thick underneath a nickel plate of 0.2 to about 2 mils thick followed by a linal chromium plate of 0.005 to about 0.1 mil thickness to give extraordinary outdoor corrosion protection to such metals as steel, aluminum and its alloys, zinc and its alloys, magnesium vand its alloys, copper and brasses.

The following outdoor exposure results obtained in an industrial atmosphere of a northern mid-western city of the United States in an area heavily contaminated with the smoke from trains, illustrate the magnitude of the improvement in corrosionprotection obtained with the use of the composite coating of this invention.

In a series of 6" x 4" steel panels plated with about 0.1 mil of chromium plate deposited from a 130 F.- 150 F. warm chromium plating bath (32-40 oz./gal. CrO3, and a ratio of Cr03 to SO.,= of about 125 to 1 to about 200 to 1) followed by a nickel plate of only 0.5 mil thickness, and a bright final chromium plate of 0.01 to 0.03 mil thick, the outdoor corrosion protection chromium plate. When the latter conventional coatings had already shown from 100 to 500'rust spots per square foot, the former still had no rust spots on most of the panels, with only an occasional 1 or 2 rust spots on some Furthermore, the underside (where dew condensation is most pronounced) of these outdoor exposed panels (exposed at 45 angle) wereA also free from corrosion whereas the conventional panels without the approximately 0.1 mil chromium underneath the nickel plate were very rusty at the end of one year outdoor exposure. 'f

Excellent results with the above described composite coating of chromium-nickel-chromium were also obtained in such accelerated tests as salt-spray, acetic ac id modiiied salt-spray (salt-spray with 5% salt solution containing acetic acid to give a pH of 3.2), and Corrodkote. `Corrodkote is an accelerated corrosion test employing a corrosive coating applied to the chrome plated surface, the coating consisting of kaolin mixed with a solution of ammonium chloride, ferrie chloride and cupric nitrate, and then this coated surface is submitted to humidity This test procedure is described more completely in Proceedings of the American Electroplaters Society, vol. 43, pp. 50-55` (1956). When the Corrodkote test was usedwith humidity instead of 90% humidity conditions, the above described chromiumnickel-chromium coating gave excellent results, whereas,

conventional coatingsof 1.5 mil thickness of nickel or copper-nickel covered with the same nal thickness Vof l phere of a northern mid-western city ofthe United States,

0.2 mil of nickel plate.

1in an area heavily contaminated with thel smoke from trains, especially during the late fall, winter and early `spring months when corrosion failure of chrome plate (the usual copper-nickel-chromium plate or nickel-chromitically stopped' on reaching the underlying chromium plate of at least 0.03 mil thickness. -If the underlying ichromium plate was below about 0.02 mil thickness', usually erratic results were obtained, in that most often `no dramatic enhancement of corrosion protection resulti ed from the presence of such a thin chromium plate under-v neath the nickel plate, or atleast the enhancement was not significant enough to be Worth the extra steps of applying the thin (0.01-0.02 mil) intermediate chromium plate. Apparently when the underlying chromium plate is only about 0.01 to 0.02 mil thick, it is most often penetrated too quickly before the exposed chromium can recover its passivity. The surface of the underlying chromium plate had been necessarily rendered active by plating it in a low pH nickel strike before the regular` nickel plate of 0.2 mil or thicker was applied. Thus, if` thev chromium plate exposed underneath the corrosion pit in the nickel is of such a thickness (at least about 0.03 mil) that it can become passive before it is penetrated, the corrosion cell set up by the presence of a lilm or layer of weakly acidic or saline electrolyte on top of the porous nal thin chromium plate will tend to develop a-,corrosion pit which tends to move laterally in the ynickel instead of downward, thus greatly superior protection of the metal underneath the lower layer of chromium is accomplished without any blistering effects or unsightly corrosion products because of the very slow corrosion i attack of nickel compared, for example, to copper or zinc, or iron. This appears to be the explanation of the dramatic improvement in corrosion protection obtained when the chromium is at least 0.03 mil thick underneath at least i Figures 1-5 are illustrative diagrams of the corrosion cell involved in outdoor exposure of nickel-chromium plate in an industrial or marine atmosphere. In Figures 1 and 5 are shown the diagrams of the corrosion cell involved when the `compositefchromium-nickel-chromium plate of this invention is on steel exposed in a damp industrial or marine atmosphere. Figure 2 illustrates the than withy much thicker nickel plate carrying the same Vfinal chromium plate, but having no chromium plate of a thickness of about 0.03 mil to r0.2 mil underneath the nickel plate. Figure 5 illustrates the case when the final thin chromium plate is of the usual 0.005 to 0.02 mil thickness and shows the appearance of the corrosion pit after 1 year of exposure to an industrial atmosphere. Figure 1 shows the typical appearance after 1 year exposure in an industrial atmosphere when the thickness of the final chromium plate is in the range of 0.03-0.05 mil, and was plated from a 120" F.-l40 F. hexavalent chromium bath. l

The outdoor corrosionv protection afforded to steel, iron, aluminum, zinc, etc. by the composite coating of, for example, 0.06 to 0.2 mil of chromium from a 120- l80 F. warm hexavalent chromium plating bath underneath 0.5 to l mil of bright nickel followed by 0.03 mil of final bright chromium from a 130 F. her/.avalent chrov, mium bath, is superior to the same nickel plate with a case when no chromium plate 'is underneath the nickel plate. Figure 3 illustrates the case when the underlying chromium plate is very thin, i. e., less than about 0.2 mil. Fig. 4 shows a thick nickel plate on the basis metalwith a conventional chromium overlayer. In each of the figures a corrosion pit is shown which started at a pore designated a in the tinal thin chromium plate A. The corrosion pit is the type that results when this chromium plate is covered with a layer of weakly acidic or saline electrolyte, which sets up a galvanic corrosion cell, with a passive tinal chromium plate as cathode, indicated by negative electrical signs in the diagrams, and the metal (nickel) exposed in the pore of the iinal thin chromium as anode, indicated by positive electrical signs. if` the nickel plate is thin, e. g. 0.2 to,y 0.5 mil, then it will not take long before the nickel plate is penetrated and the more readily corrodible iron is attacked when there is no underlying chromium plate, or very thin chromium plate, as may be seen in Figures 2 and 3, respectively. With thicker nickel plate (1.5 to 2 mil), it will take much longer for the corrosion pit to reach the iron or steel, as illustrated in Fig. 4. However, with a chromium plate of about 0.03 to 0.2 mil underlying the nickel plate, as shown in Fig. l, the corrosion pit tends to go, laterally in the nickel plate on reaching the underlying chromium plate instead of downwarchthus protecting the more readily corrodible base metal (Fe, Al, Zn, Mg, Cu, brass, etc.) underneath, in a much more4 etective mannery total iinal chromium plate equal to the underlying and overlying chromium plate thickness. VFurthermore, such thick final chromium coatings must be polished to he bright and are very impractical to use for bright, high lustre coatings because of the excessive costs to polish them to a high lustre, whereas, by using a semi-bright or dull chromium plate of about 0.08 to 0.2 mil thickness preferably from Warm (120 F.-180 F.) hexavalent chromium plating baths, it is possible to cover this plate with a bright deposited nickel plate that will brighten up the dull or semi-bright chromium to full bright lustre for the final thin bright chromium plate. Thus, by distributing the thicker values (0.1 kto 0.2 mil) of the chromium plate underneath and-on top of a nickel plate, it is not only possible to obtain a minimum of stress-cracking of the final bright chromium plate, but also to avoid polishing a thick semi-bright or dull chromium plate ofvminimum stress-cracks, and there is also obtained dramatically superior outdoor corrosion protection which is far superior to bright nickel plate of double the total thickness with the same iinal thin bright chromium plate. The main reason for this lies in the favored laterall corrosion of the nickel plate sandwiched between the chromium plate of .0.03 to 0.2 mil thickness and the final thin bright chromium deposit, as already explained, and diagrammatically illustrated in Figures l and 5.

l This sequence of chromiumv plate of 0.03 to 0.2 mil followed by 0.2 to 2 mils of nickel followed by a final chromium plate of 0.005 to 0.1 mil thickness, used directly on zinc, or its alloys, aluminum or its alloys, or on magnesium, or on copper or brass provides the same excellent protection in outdoor exposure that is given to steel. The aluminum can often be plated directly, or it can first be anodized or partially anodized before plating, or

l covered with a zinc iilm (as from an immersion zincato solution or iluoboratesolution), or covered with an im-j mersion zinc iilm or Zinc electroplate, and a copper plate or brass plate, before applying the above described chro-` mium-nickel-.chromium plate. The composite chromiumnickel-chromium plate of this invention applied to alumiventional copper-nickel-chromium coatings.

or zinc alloys such as zinc base die-cast can satisfactorily num or aluminum alloys as already described gives far more superior corrosion protection than the usual heavy copper-nickel-chromium coatings of at least a total of 2 to 3'mils thickness applied to aluminum, and can be used on high strength aluminum alloys that do not yield a bright anodized finish.y In this way aluminum and its al.- loys can be given theY high lustre of chrome plate but with excellent outdoor corrosion resistance instead of the usual corrosion blistering resulting from the present `con- Zinc metal irst be copper plated or brass plated or nickel plated be fore starting the seque/nce of the above described compos ite chromium-nickel-chromi um plate.

The preferred chromium plate for use underneath the' nickel plate is that obtained from warm hexavalent chrc mium baths operated at about F. to 180 F., whichY 5 yield chromium plate with minimum stress. The underlying chromium plate may be dull, semi-bright or bright, though it is preferred to be semi-bright or even dull, and actually excellent results are also obtained with lower temperature 60 F.-70 F. (even refrigerated to 50 F.) chromium plating baths, especially with such baths as the chromium plating bath described in French Patent 754,299, which can be used to plate zinc die-cast directly. Alternatively the underlying chromium plate may be deposited by a contact process such as by brush plating as in the Dalic process as describedin Electroplating, vol. 6, No. 4, p. 131 (1953). The presence of an overlying nickel plate of at least 0.2 mil thickness tends t make the differences between various chromium plates (as obtained from warm versus cold baths) less important than when compared without the overlying nickel plate. Nevertheless, the underlying chromium plate should be of minimum stress for best results.

In depositing the thicker underlying chromium plate on complex shaped articles (articles with deep recesses), it is best to use conforming or auxiliary anodes to obtain optimum distribution of the rst chromium plate. Then after rinsing thoroughly, the parts can be re-racked without using conforming anodes for the subsequent nickel and final chromium plate. On simpler shaped articles the use of conforming or auxiliary anodes for the irst chromium plate are of course not needed. In any case, after the first chromium plate of at least 0.03 mil to about 0.2 mil is deposited, the articles are thoroughly rinsed including a dip or preferably an agitated soak in a water solution of a reducing agent capable of reducing hexavalent chromium to trivalent chromium, such as compounds containing sulfur with a valence less than +6 in anion form including sodium bisulte, sodium sulfite, hydrosulte and thiosulfate. Other reducing agents such as aqueous solutions of hydrogen peroxide, sugars or ferrous salts can be used to reduce the last traces of chromic acid on'the work or rack that might not have been rinsed off. In addition sequestering agents capable of preventingthe precipitation of Cr(OH)3 in the reducing solutions is advantageously incorporated such as sodium gluconate, nitrilotriacetate, ethylene `diamine tetra-acetates, etc. After further rinsing, the chromium plated articles are given a low pH nickel strike'for example, from nickel strike baths such as the following: 24 oz./gal. NiClgl-LO with 5% to 10% by volume of hydrochloric acid; or 12 oz./ gal. NiSO4.6H2O-l12 oz./ gal. NiCl2.6l-l20 and 5% to 10% by volume of hydrochloric acid, with or without boric acid present; or from nickel sulfate baths containing to 20% sulfuric acid. After about a 1 to l0 minute nickel strike plate is made at cathode cur rent densities of 10-100 amps/sq. ft., the articles are transferred with or without rinsing to regular nickel plating baths of the Watts type, high chloride type, iluoborate or sulfamate type, etc. These nickel baths can be used with addition agents to give semi-bright or bright nickel plating or a series of nickel baths can he used combining dull nickel, semibright nickel with nal bright nickel. It is preferred to use bright nickel or a combinationof a semi-bright nickel and bright nickel. After obtaining a bright surface either by bufling the semi-bright or by bright nickel plating, the nickel surface is given the usual iinal thin bright chromium plate. It is preferred to use a final bright chromium plate from a 120 F.-l40 F. bath (preferably about 130 F.) for a thickness of 0.03 to about 0.08 mil. In general, however, the thickness for the final chromium plate need not be greater than about 0.03 mil for very excellent results, and can be the usual bright chromium plate4 from the room temperature to 140 F. baths. p l

The preferred underlying chromium plate of 0.03 to 0.2 mil thickness of semi-bright or dull chromium of minimum stress and of maximum corrosion resistance, is best brightened, as already suggested, by the use of semibright high-leveling nickel plating followed by fully bright nickel, or by fully bright nickel alone, and this way laborious and expensive buiiing of a thick"ser'1ibright-of dull chromium surface is avoided. Not onlyis'it expensive to buff or polish thick chromium plate, as valready mentioned, but it also causes a significant lossv in corrosion resistance of the chromium plate itself. Whereas, by covering the unpolished underlying chromium plate with a nickel coating of equal or greater'thicknessV than the chromium plate, the eiect of any residual stresses in the underlying chromium plate is decreased by beingsandwiched between more ductile metal. i

T he above described sequence 'of chromium plate followed by nickel followed by chromium can be used on stainless steel with beneficial elects. While stainless steel, especially chromium plated stainless steel gives superb results in outdoor exposure, it nevertheless is diicult to obtain a high lustre on stainless steel by polshing because of its work-hardening. qualities. If stainless steel is given a nickel strike and then brightened by'application of bright nickel plate, it loses its great resistance to rust ing because the use of the low pH nickel strike results in removal of the passive film of the stainless steel. Whereas, if the stainless steel is plated with the sequence of chro niium-nickel-chromium as given above, that is, chromium plated iirst from the hexavalent chromium bath with a thickness of about 0.03 mil chromium and then given a low pH nickel strike and bright nickel plated with about 0.2 to 0.5 mil nickel, and given a nal thin (0.0l-0.03y mil) bright chromium plate, it is possible to brighten up the stainless steel with bright nickel to a high lustre without a serious loss of corrosion resistance. The term stainless stee is intended in its'conventionally understood sense.

In the case of brass articles, it is not as important, because of the excellent corrosion resistance of nickelchromium plated brass, to usetlie above described sequence of chromium-nickel-chromium plate,- as it is for. steel, aluminum, magnesium, and zinc, nevertheless, it makes possible even superior outdoor corrosion protection with thinner nickel plate than is usually used. Thus, instead of 0.5 to 0.8 mil nickel plate, only about 0.2 mil nickel plate sandwiched between coatings of 0.03 mil of chromium is all that is needed.

In general, beforeA the application of the sequence of the above described composite chromium-nickel-chromium plate, electroplates of copper, brass, Zinc, or nickel could be used on the basis metal. In each case, outstanding corrosion protection is conferred to the underlying metal by the above described composite chromium-nickelf chromium electroplate sequence.

The following examples further illustrate the use of thel composite chromium-nickel-chromium electroplate of this invention to obtain bright chrome finished articles of outstanding corrosion resistance to outdoor exposure'.

EXAMPLE I Steel basis metal Polished steel suitable for use as automobile bumpers is cleaned in the conventional manner and after a final alkaline anodic cleaner, rinsed, acid dipped or anodically etched, and then given a chromium plate of 0.1 to 0.2 mil thickness from the following hexavalentv chromium bath: y

After the initial chromium plating is iinished, the bumpers are thoroughly rinsed, and given an agitated soak dip in a 5% solution of sodium sulte, rinsed thoroughly in Water and transferred to a low A OZ./gal NiCl2.6H2O (or lower nickel chloride and higher nickel sulfate content) oZ./gal l2 H3BO3 oz./gal 4l 5%-l0% by volume of concentrated hydrochloric acid Room temperature Cathode .current density ampsJsq. ft 20-30 Plating time minutes 3-5 After removal from the nickel strike, the bumpers are then transferred to a ductile semi-bright type Watts nickel plating bath containing an addition agent selected from the class consisting, for example, of bromal, chloral hydrate and S-methoxy coumarimand plated with about 0.3 mil to 1 mil of semi-bright nickel and transferred to a bright nickel plating bath containing a brightener selected from the class consisting of benzene sulfonamides and sulfonimides especially o-benzoyl sulmide, together with an aliphatic unsaturated compound or compounds, and plated with about 0.2 to 0.3 mil of bright nickel, then rinsed and transferred to a chromium bath of approximately the following composition:

CrO3 oz./gal 33 S04 oz./gal 1.5-2 Temperature F 131 Cathode current density amps./sq. ft. 285-325 and plating a thickness of bright chromium of 0.03 to 0.05 mil.

EXAMPLE II Steel basis metal Polished steel bumper guards are put through the same steps given in Example IV except that the first chromium plating bath is operated at 130 F. and about 300 amps/ sq. ft. and a chromium plate of about 0.1 mil is applied,

and after the nickel strike is applied, the articles are transferred directly to the bright nickel bath and given about a 1 mil thick nickel plate, and then a iinal chromium plate of 0.03 to 0.06 mil thickness from a 120 F.- 140 F. hexavalent chromium bath.

EXAMPLE In Aluminum basis metal lAfter employing the usual methods of cleaning aluminum and its alloys, the clean aluminum is given a dip in an alkaline zincate solution for seconds to about 60 seconds, then rinsed and given a copper strike from a low metal low free cyanide copper bath and using low cathode current densities (about 5 to l0 amps/sq. ft), and the copper plating continued until about 0.1-0.3 mil of copper is deposited and then after rinsing, the chromiumnickel-chromium electroplate sequence is applied as given in Example II above.

EXAMPLE IV Zinc base die-cast metal above. The Bornhauser chromium plating bath is as follows:

Cr03 oZ./gal 53 NaOH oz./gal 7.7

8 Cr203 oz./gal 1.0 H2504 oz./gal 0.1 Temperature F' 60-70 Current density amps/sq. ft ZOO-800 Articles plated in accordance with this example are suitable for use outdoors or as laboratory hardware, such as faucet handles, etc.

EXAMPLE V Magnesium and magnesium-alloy basis metals After the usual methods of cleaning and preparation of the surface of the magnesium articles is completed, for example, according to directions given in Bulletin No. DM108b of the` Magnesium Department of the Dow Chemical Co., Midland, Michigan, and the magnesium has been given a zinc immersion coating and a copper plate of about 0.1 to 0.3 mil thick, it is then given the chromium-nickel-chromium electroplate sequence as described in Example II above.`

The plated surfaces resulting from the process above specified in Examples I and II were subjected to outdoor corrosion in the northern mid-west section of the United States, and upon inspection at the end of one year showed no evidence of corrosion. Test panels of the same type metal given a conventional copper-nickel-chromium coating with the nickel thickness being similar to that specied in Examples I and II when subjected to the same outdoor corrosion showed from -500 rust spots per square foot.

The aluminum, zinc and magnesium plated in accordance with the procedures set forth in Examples HI, IV and V respectively, when subjected to the Corrodkote test, which simulates one year of outdoor exposure, show no evidence of blistering. F or the best corrosion protection of steel articles that will undergo deformation in service, as for example, automobile bumpers and bumper-guards, etc., it was found that by using an underlying plate of zinc or cadmium or alloys thereof which are anodic to steel of about 0.1 to 0.5 mil thickness beneath the chromiumnickel-chromium plate, that no rusting occurred with severe distortion of the plated steel articles on outdoor eX- posure for extended periods of time. The zinc, cadmium, or anodic-to-steel alloy thereof can be coated directly with the chromium-nickel-chromium plate of this inventiornor a 0.1-0.3 mil coating of copper or yellow brass can be, and sometimes is preferably, plated directly on the Zinc, cadmium or anodic-to-steel alloy coating prior to the plating of the chromium-nickel-chromium plate. In this aspect of the invention the electroplate which underlies the chromium-nickel-chromium coating of this invention may be a single or a composite coating which is at least in part anodic-to-steel, for example a zinc coating directly on the steel surface which is overlaid with an adherent copper coating. However, even without this additional anodic-to-steel plate underneath the lower thick chromium plate, it is surprising how little rusting occurs with severely distorted chromium-nickel-chromium plated steel articles.

EXAMPLE VI Steel of the type used in Example I, after cleaning is given a zinc plate of 0.2 to 0.5 mil thicknesgfrom a zinc cyanide bath, and then a copper strike of 0.1-,0.2 mil thickness from a Lcyanide copper bath. This surface is then chromium-nickel-chromium plated as in Example I.

This produces a composite plated surface that even when severely distorted, has excellent corrosion resistance.

Vt/hat is claimed is:

l. A laminated corrosion-protective final coating on a metal base susceptible to atmospheric corrosion comprising a layer of chromium having a thickness in the range of about 0.03 to about 0.2 mil, nickel overlying said chromium and adherent lthereto and having a thickness in the range of about 0.2 mil to about 2 mils, and chromium overlying said nickel and adherent thereto and havinga -.06 to about 0.15 mil, electroplating on said nickness in the range of about 0.005 mil to about 0.1 2. A laminated corrosion-protective inal electroplate on a metal base susceptible to atmospheric corrosion comprising a layer of adherent electroplated chromium having a thickness in the range of about .06 to about .15 mil, electroplated nickel overlying said chromium and adherent thereto and having a thickness in the range of about 0.5 to 1 mil, and electroplated chromium overlying said nickel and adherent thereto and having a thickness in the range of about 0.03 mil to about 0.08mil.

3. A laminated corrosion-protective nal electroplate on a metal base of the class consisting of iron, steel, stainless steel, copper, brass, aluminum and its alloys, magnesium and its alloys and zinc and its alloys consisting of electroplated chromium having a thickness in the range of about 0.03 to about 0.2 mil, electroplated nickel overlying said chromium and adherent thereto and having a thickness in the range of about 0.2 mil to about 2 mils, and electroplated chromium overlying said nickel and adherent thereto and having a thickness in the range of about 0.005 mil to about 0.1 mil.

4. A coating in accordance with claim 1 wherein said nickel plate comprises a semi-bright portion overlaid by a bright portion.

5. A coating in accordance with claim 1 wherein said nickel plate is substantially all bright nickel.

6. A composite corrosion-protective electroplated final coating on steel consisting of a first electroplate, at least a part of which is anodic-to-steel and is an electroplated metal 4selected from the group consisting of zinc, cadmium and the anodic-to-steel alloys thereof, adherent electroplated chromium overlying said rst electroplate and having a thickness in the range of about 0.03 to 0.2 mil, electroplated nickel overlying said chromium and adherent thereto and having a thickness in the range of about 0.2 mil to about 2 mils, and electroplated chromium overlying said nickel and adherent thereto and having a thickness in the range of about 0.005 mil to about 0.1 mil.

7. A coating in accordance with claim 1 wherein said metal base is steel.

8. A coating in accordance with claim 3 wherein said metal base is aluminum and its alloys.

9. A coating in accordance with claim metal base is zinc and its alloys.

10. A coating in accordance with claim 3 wherein said metal base is magnesium and its alloys.

11. A method for electroplating from aqueous solutions a corrosion-protective composite coating on a metallic surface susceptible to atmospheric corrosion which comprises the steps of electroplating on said surface adherent chromium having a thickness in the range of about 0.03 mil to about 0.2 mil, electroplating on said chromium adherent nickel having a thickness in the range of about 0.2 mil to about 2 mils, and thereafter electroplating on said nickel adherent chromium having a thickness in the range of about 0.005 to 0.1 mil.

12. A method for electroplating from aqueous solutions a corrosion-protective composite coating on a metallic surface susceptible to atmospheric corrosion which comprises the steps of electroplating on said surface adherent chromium having a thickness in the range of about chromium adherent nickel having a thickness in the range of about 0.5 to 1 mil, and thereafter electroplating on said nickel adherent chromium having a thickness in the range of about 0.03 mil to about .08 mil.

13. A method for electroplating from aqueous solutions a corrosion-protective composite coating on a metal base of the class consisting lof iron, steel, stainless steel, copper, brass, aluminum and its alloys, magnesium and its alloys and zinc and its alloys which comprises the steps of electroplating on said surface adherent chromium having a thickness in the range of about 0.03 mil to about 0.2 mil, electroplating on said chromium adherent nickel 3 wherein said having a thickness in the range of about 0.2- mil to about 2 mils, and thereafter electroplating on said nickel adherent chromium having a thickness in the range of about 0.005 to about 0.1 mil.

14. A method for electroplating from aqueous solutions a corrosion-protective composite coating on a metal base of the class Consisting of iron, steel, stainless steel, copper, brass, aluminum and its alloys, magnesium and its alloys and Zinc and its alloys which comprises the steps of electroplating on said surface adherent chromium lhaving a thickness in the range of about .06 to about .15 mil, electroplating on said chromium adherent nickel having a thickness in the range of about 0.5 to l mil, and thereafter electroplating on said nickel adherent chromium having a thickness in the range of about .03 mil to about .08 mil.

15. A method for electroplating from aqueous solutions a corrosion protective composite coating on a metallic surface susceptible to atmospheric corrosion which comprises the steps of electroplating on said surface an adherent chromium plate having `a thickness in the range of about 0.03 mil to about 0.2 mil, rinsing said chromium-coated surface in an aqueous rinse solution containing a reducing agent capable of reducing the hexavalent chromium ion to the trivalent chromium ion, electroplating on said rinsed chromium coating an adherent nickel plate having a thickness in the range of about 0.2 mil to about 2 mils, and thereafter electroplating on said nickel pl'ate an adherent chromium plate having a thickness in the range of about 0.005 to about 0.1 mil.

16. A method in accordance with claim l5 wherein said aqueous rinse solution contains in addition to said reducing agent a sequestering Iagent capable of preventing precipitation of chromic hydroxide.

17. A method for electroplating from aqueous solutions a corrosion protective composite coating on a metallic surface susceptible to atmospheric corrosion which comprises the steps of electroplating on said surface a first electroplate, at least .a part of which is anodic-to-steel and is an electroplated metal selected from the group consisting of zinc, cadmium and the anodic-tO-steel alloys thereof, electroplating on said rst electroplate adherent chromium having a thickness in a range of about 0.03 to about 0.2 mil, electroplating on said chromium adherent nickel having a thickness in the range of about 0.2 to about 2 mils, and thereafter electroplating on said nickel adherent chromium having .a thickness in the range of about 0.005 to about 0.1 mil.

18. A method for electroplating from aqueous solutions a corrosion protective composite coating on a metallic surface susceptible to atmospheric corrosion which comprises the steps of electroplating on said surface a first electroplate, at least a part of which is anodicto-steel and is an electroplated metal selected from the group consisting of zinc, cadmium andthe anodic-tosteel alloys thereof, electroplating on said iirst electroplate adherent chromium having a thickness in a range of about 0.06 to about 0.15 mil, electroplating on said chromium adherent nickel having a thickness in the range of about 0.5 to about 1 mil, and thereafter electroplating on said nickel ladherent chromium having a thickness in the range of about 0.03 to about 0.08 mil.

References Cited in the le of this patent UNITED STATES PATENTS 1,615,585 Humphries Jan. 25, 1927 2,093,428 Ford et al Sept. 21, 1937 2,147,407 Huston et al Feb. 14, 1939 2,188,399 Bieber Jan. 30, 1940 2,336,568 Pray Dec. 14, 1943 FOREIGN PATENTS 684,434 Great Britain Dec. 17, 1952 

1. A LAMINATED CORROSION-PROTECTIVE FINAL COATING ON A METAL BASE SUSCEPTIBLE TO ATMOSPHERIC CORROSION COMPRISING A LAYER OF CHROMIUM HAVING A THICKNESS IN THE RANGE OF ABOUT 0.30 TO ABOUT 0.2 MIL, NICKEL OVERLYING SAID CHROMIUM AND ADHERENT THERETO AND HAVING A THICKNESS IN THE RANGE OF ABOUT 0.2 MIL TO ABOUT 2 MILS, AND CHROMIUM OVERLYING SAID NICKEL AND ADHERENT THERETO AND HAVING A THICKNESS IN THE RANGE OF ABOUT 0.005 MIL TO ABOUT 0.1 MIL. 